Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of a resistive material measuring about 50 .mu.m.times.50 .mu.m, is located in a chamber filled with ink supplied from a reservoir. A nozzle plate, comprising a plurality of nozzles or fine openings, with each nozzle associated with a resistor element, defines one side of the chamber. Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
The tight tolerances of the nozzles (typically 40 to 50 .mu.m diameter) require that the ink not clog the nozzles. Further, repeated firings of the resistor elements, which must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor elements. This is unique to thermal ink-jet printers and is known as kogation.
The ink composition must be capable of interacting with the print medium, especially paper, to penetrate the paper without undue spreading and a subsequent decrease in print quality. Finally, the printed characters must dry rapidly and be waterfast and smear resistant.
Inks are known which possess one or more of the foregoing properties. However, few ink compositions are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations. Accordingly, investigations continue into developing ink formulations which have improved properties and which do not elevate one property at the expense of the others.
In a related patent application entitled, "Waterfast Ink Formulations with a Novel Series of Anionic Dyes Containing Two or More Carboxyl Groups", Ser. No. 07/398,018, filed Aug. 24, 1989, and owned by the same assignee as the present application, inks are disclosed which include a dye corresponding to the formula in the free acid state of ##STR2## wherein W is COOH,
X is H or COOH, PA1 Y is H, COOH, or SO.sub.3 H PA1 Z is H, COOH, or SO.sub.3 H, and PA1 R is H, CH.sub.2 COOH, or CH.sub.2 CH.sub.2 COOH; PA1 X is H or COOH, PA1 Y is H, COOH, or SO.sub.3 H, PA1 Z is H, COOH, or SO.sub.3 H, and PA1 R is H, CH.sub.2 COOH, or CH.sub.2 CH.sub.2 COOH; PA1 W and X are COOH, PA1 Y is H, PA1 Z is SO.sub.3 H, PA1 R is H; PA1 W and X are COOH, PA1 Y and Z are H, PA1 R is H; PA1 W and X are COOH, PA1 Y is SO.sub.3 H, PA1 Z is H, PA1 R is H. PA1 1) reduction of nozzle clogging, PA1 2) reduction of pen bearding, PA1 3) reduction of ink bronzing on paper, PA1 4) greater pH flexibility, and PA1 5) control of hue characteristics of ink. PA1 1 1 wt % Dye I PA1 1.1 wt % Dye II PA1 7.5 wt % 2-pyrrolidone PA1 0.4 wt % buffer (0.2 wt % TRIS and 0.2 wt % BORAX) PA1 0.07 wt % phosphate anion PA1 0.1 wt % EDTA PA1 0.3 wt % PROXEL GXL PA1 the balance water.
provided that there are at least two COOH groups and that the number of COOH groups is equal to or greater than the number of SO.sub.3 H groups. The concentration of the dye in the inks ranges from about 0.5 to 20 wt %. The inks further include up to about 30 wt % of a water-soluble organic solvent for the dye and the balance water.
While the above inks are suitably employed in thermal ink-jet printers and evidence excellent waterfast printing on paper, efforts continue to develop further improvements in ink formulations.